The present invention relates to the use of decision feedback equalization in communication systems which employ coded modulation.
Decision feedback equalization is a preferred technique for compensating for intersymbol interference, or ISI, engendered in certain kinds of transmission channels. In a traditional decision feedback equalization arrangement, an estimate of the ISI component of a received signal representing a transmitted signal point is subtracted from the received signal prior to a decision being made as to which signal point was transmitted. The estimate is generated as a function of a plurality of previous decisions--typically, as a linear combination of same.
Using decision feedback equalization in a communication system which employs coded modulation, such as trellis coded modulation, gives rise to at least one difficulty. Specifically, the process of decoding the transmitted signal points involves a delay of many signaling intervals due to the use of maximum likelihood decoding in, for example, the Viterbi decoder that is typically used. However, the decision feedback equalizer, or DFE, requires immediate past decisions in order to estimate the ISI component. U.S. Pat. No. 5,056,117 issued to Gitlin et al. on Oct. 8, 1991 discloses a joint Viterbi decoder/DFE arrangement which addresses this problem by providing a separate DFE for each state of the Viterbi decoder. The tentative signal point decisions along the surviving path of each state are used in the associated DFE to generate an equalized signal for use in calculating the so-called branch metrics for trellis branches emanating from the state in question.